An optical disc reproduction device for reproduction of a reproduction-only optical disc is known. When reproduction starts, this optical disc reproduction device drives a laser diode causing it to emit a laser beam. A diffraction grating divides this laser beam into a main beam, which is a zero-order light beam, and two side beams, which are ±1-order light beams, the laser beam is then irradiated onto a polarizing beam splitter. Most of the aforementioned laser beam is P-polarized light, and the polarizing beam splitter is characterized in that it reflects this P-polarized light component and lets a S-polarized light component, which is polarized in a direction orthogonal to the P-polarized light component, pass through. Therefore, the laser beam that is irradiated onto the polarizing beam splitter is reflected by the polarizing beam splitter, is made parallel by a collimator lens, and becomes incident on an object lens. The object lens, causes the incident laser beam to converge into a beam spot having a specified diameter, and irradiates that beam onto an optical disc. When doing this, the main beam is irradiated on the disc so that the center of the beam coincides with the center of the recording track, the +1 order side beam is irradiated on a position that it is shifted ¼ track portion toward the outside of the recording track, and the −1 order side beam is irradiated on a position that is similarly shifted ¼ track portion toward the inside of the recording track. The diffraction grating has spectroscopic characteristics that irradiate each beam in such a state.
Next, when each of the beams is irradiated as described above, reflected light from each beam occurs. Each of the reflected light beams pass through an object lens and collimator lens and become incident on a polarizing beam splitter. That is, the laser beam of the P-polarized light component is reflected by the optical disc such that the light path is reversed, and becomes incident on the polarizing beam splitter as reflected light of an S-polarized component. As described above, the polarizing beam splitter allows the S-polarized light component to pass through. Therefore, each of the reflected light beams that are incident on the polarizing beam splitter pass through that polarizing beam splitter and are irradiated onto a photodetector. The photodetector comprises a first photodetector that receives the reflected light of the main beam, and second and third photodetectors that receives the reflected light of the ±1 order light, respectively. The first photodetector is a 4-way photodetector having four equally divided light-receiving areas A to D, with the light axis of the reflected light of the received main beam in the center, and generates light quantity detection signals that correspond to the amounts of light of the reflected light that is received by each of the light-receiving areas A to D, and supplies these signals to a signal processing system and focus control system. The signal processing system adds all of the light quantity detection signals from each of the light receiving areas A to D to reproduce and output an RF signal that indicates recorded data that is recorded on the optical disc. The focus control system adds each of the light quantity detection signals from the light receiving area A and light receiving area C that are located on the diagonal line, and adds the light quantity detection signals from the light receiving area B and light receiving area D, and by detecting the difference between these added signals, detects focus error by the so-called astigmatic method, and performs focus control of the optical pickup device in accordance to this focus error. The second and third photodetectors receive the reflected +1 order light and the reflected −1 order light, respectively, and supplies the light quantity detection signals that correspond to these reflected light quantities to a tracking control system. The tracking control system detects the difference between each of these light quantity detection signals, and detects tracking error by the so-called three-spot method, then performs tracking control of the optical pickup device in accordance to this tracking error. By doing this, it is possible to reproduce recorded data that is recorded on the optical disc with the laser beam always focused and with the main beam accurately tracing the recording track.
[Patent Document 1] JP2004-318958